Semiconductor integrated circuits are manufactured by forming patterns in material layers on an appropriate substrate, such as a silicon substrate. The various material layers can be patterned using lithography, wherein a mask layer is used for patterning a material layer. One type of semiconductor lithography involves exposing a patterned mask on the substrate, and using a light source to expose portions of a resist spinned on the substrate, transferring the mask pattern to the resist. The resist is then developed, and the resist is used as a mask while open regions of a material on the substrate are etched away.
The fabrication of an integrated circuit using lithography generally requires device features of precisely controlled size and position to be formed in the mask layers and in the material layers in order to meet the performance targets of the completed integrated circuit. As the size of features in integrated circuits steadily shrinks, control over the dimensions of small features becomes more important. Lithography becomes more difficult, because of operating in the nonlinear subresolution regime. Several phenomenon of optics can prevent the exact transfer of a mask pattern onto a substrate. Therefore, the manufacture of mask layers and material layers requires continual monitoring and measurement to ensure that the dimensions of the patterns being formed are within acceptable ranges.